The light-emitting diodes (LEDs) of the solid-state lighting elements have the characteristics of low heat generation, long operational life, small volume, quick response and the light with a stable wavelength range, so the LEDs have been widely used in various applications. Recently, efforts have been devoted to improve the luminance of the LED in order to apply the device to the lighting domain, and further achieve the goal of energy conservation and carbon reduction. In order to apply LED device to daily life use, such as lighting, various control apparatus are designed for different applications such as luminance controller, light sensor, traffic light controller, automobile lighting, power supply circuit, and so on.
The stability of the light characteristics is also an important issue. Typically, the LED is sensitive to the environmental temperature which means that the higher the ambient temperature, the lower the light emitting efficiency of LED. Take a blue LED and a red LED as examples, as FIG. 1 shows, while the environmental temperature increases from 25° C. to 100° C., the light emitting efficiency of blue LED decreases to 90% and the efficiency of red LED drops to 65%.
In another aspect, the stability of luminance per watt is also an important issue. While dimming function is added into the LED control circuit to change the light intensity by controlling the current density, the change of current density also changes the luminance per watt. Take a blue LED and a red LED as examples, as FIG. 2 shows, while the operating current decreases from 20 mA to about 3 mA, the luminance per watt of blue LED increases from 75% to about 100% and the luminance per watt of red LED also increases from 55% to 90%. Moreover, when the operating current decreases from about 3 mA to 0 mA, the luminance per watt of blue LED increases from 90% to about 100%, but the luminance per watt of red LED decreases from 100% to 90%. While a blue LED and a red LED are put together accompanied with yellow phosphor to emit a predetermined white light, the temperature is increased due to long time use so the light intensity is decreased and the correlated color temperature (CCT) is also changed. Once the operating current of LEDs is changed from 20 mA to 2 mA, the CCT of the white light shifts in an unexpected way.